As is exemplified by precoated metal sheets, a method is known in which, when continuously applying a multilayer film of two or more layers onto a steel sheet or other flat sheet, and baking the film onto the sheet by rapid heating within about 90 seconds, the coating material is applied and baked for each layer after which this is repeated for the subsequent layers. In this method, however, since it is necessary for products to pass through the line a plurality of times in the case of ordinary application and baking lines, work efficiency is poor and the amount of energy required for baking becomes excessively large. In addition, if a line is attempted to be composed in which application and baking of all layers is completed in a single pass through the line, the line becomes excessively long, which together with resulting in poor production efficiency, leads to excessively high production equipment costs. Moreover, in the case of repeating application and baking for each layer, there are cases in which adhesion becomes poor when an upper layer is additionally formed on the uppermost lower layer that has been hardened by baking. In addition, there are cases in which the paint deteriorates when gas is absorbed through the upper layer during baking.
A known example of a technology for compensating for the aforementioned problems in the case of carrying out application and baking for each layer involves a so-called wet-on-wet application technology in which the next layer is applied before a previously applied layer is allowed to dry. Wet-on-wet technology is used as a so-called post-coating method, and normally at least 10 minutes are secured for the baking time. Since an upper layer is applied before a lower layer is allowed to dry completely, there are problems resulting from the interface being easily disturbed and the potential for entrapment of air bubbles in the vicinity of the interface.
A curtain application method is a known example of a method for simultaneously applying a plurality of layers of paint films on a flat sheet. JP 62-47075 B describes a method in which a compound layer is formed by allowing a plurality of fluid layers formed with a plurality of slit-like orifices to flow so as to mutually make surface-surface contact, and that compound layer is adhered on a moving web (flat sheet) in the manner of free-falling curtain to form a plurality of layers. This method is mainly used to produce photographic materials.
In JP 7-24401 A, a curtain application method is applied as a method for continuously applying a paint onto an object to be coated such as a steel sheet. The object to be coated is passed beneath a paint curtain that flows out and drops down from a slit-like nozzle to form a paint film by allowing the paint curtain to adhere to the upper surface of the object to be coated. The steel sheet to which paint has been applied is subsequently continuously sent to a drying oven where the solvent (volatile component) in the paint film is evaporated to bake or dry and harden the paint film.
During the baking of a steel sheet and the like, if the thickness of the paint film prior to baking is excessively thick, a phenomenon referred to as “popping” is known to occur in coated films following baking. Popping is a foam-like surface defect of a coated film surface, and appears in the form of a foam-like defect due to the formation of air bubbles within a coated film due to rapid evaporation of solvent remaining inside a coated film caused by heating during baking, which results in deformation of a previously hardened coated film surface. Its occurrence is particularly prominent in cases of thick coated films. In curtain application in which multilayer paint films are applied simultaneously, the thickness of the paint film prior to baking inevitably increases, thereby resulting in increased susceptibility to the occurrence of popping.
According to JP 7-24401 A, by making the solvent concentration of a paint film applied to an object to be coated prior to baking or drying on the side that contacts the object to be coated lower than that on the opposite side, the occurrence of popping during baking can be reduced.